1. Field of the Invention
This invention relates to an apparatus for measuring the variation with time of an angle of optical rotation of a substance such as an optically active liquid crystal having a photochromic functional group, whose angle of optical rotation is changed by an external signal such as optical excitation from outside (hereinafter referred to as "an angle-of-optical-rotation variation measuring apparatus", when applicable).
2. Prior Art
There has been known an optically active liquid crystal in the art, wherein upon application of light the photochromic functional group is optically activated; that is, the photochromic functional group is changed in structure, with the result that the optical active substance of the liquid crystal having the photochromic functional groups between the molecules is changed in the angle of optical rotation.
A light-light modulator can be formed by utilizing such a substance.
The change in the angle of optical rotation of the optically active substance occurs in the sequent steps of application of an exciting light beam, optical isomerization of the photochromic compound, high order structural change of the liquid crystal molecules and change in the angle of optical rotation.
In general, the liquid crystal shows a singular optical activity with a time delay from application of the exciting light beam.
FIG. 3 is a time chart showing the optical activity caused by an external stimulus (such as application of the exciting light beam).
As is apparent from FIG. 3, the optical active substance shows the optical activity with a delay time from the application of the external stimulus (or exciting light beam). Even after the application of the exciting light beam has been suspended, the angle of optical rotation is increased and then maintained at a certain value for a while. Then, the angle of optical rotation is gradually decreased and consequently restored to the original state.
An apparatus for measuring the behavior of rotation of polarizing plane of the specimen which is used for forming the light-light modulator is constructed as shown in FIG. 4.
In the apparatus, a polarizer 41, a specimen 40 and an analyzer 42 are arranged in this order.
A light beam linearly polarized by the polarizer 41 is applied to the specimen 40. Of the light polarizing planes of which are rotated, those which coincide in angle of optical rotation with the analyzer 42 are passed through the analyzer 42, concentrated by a lens 43, and subjected to photo-electric conversion by a detector 44.
As shown in FIG. 5, when the angle of optical rotation of the specimen reaches a value .theta. (an angle of the analyzer at which light is mainly passed through the analyzer), the output of the detector is maximum.
More specifically, the maximum output is obtained twice while the angle of optical rotation is increased to the value .theta. and decreased to the value .theta., as is apparent from FIG. 5.
The light beam (or probe light beam) applied to the specimen 40 through the polarizer 41 is constant in intensity.
When the angle (.theta.) of the analyzer 42 is set to an optional value, the time required for the angle of optical rotation of the light passed through the analyzer to reach the angle thus set can be determined from the period of time between the time instant when the external signal is inputted and the time instant when the output of the detector 44 becomes maximum. Accordingly, an optical rotatory characteristic curve can be formed according to a method in which, in the above-described apparatus, the angle .theta. of the anlayzer is set to various values, and periods of time for the output of the detector to reach the values thus set are measured.
In the above-described apparatus, the optical pulse is used for the excitation; however, it is well known in the art that instead of the optical pulse, an electric field, magnetic field or pressure may be used.
The optical rotatory characteristic curve of a specimen under examination can be obtained by using the data which are provided by operating the apparatus as described above; however, the time resolution thereof is limited by the characteristic of the detector employed. For instance, in the case where the optical detector is made up of a photomultiplier, it is impossible to perform the measurement at a speed higher than the response speed of the photomultiplier.
Furthermore, as was described above, one measurement detects a phenomenon fragmentarily with respect to time. Therefore, when the response of a specimen under test is not accurately in correspondence with an excitation thereof (i.e., when the specimen has hysteresis), reproduction of the correct profile thereof cannot be obtained.